Scientists generally agree that the answer to the riddle of the age of the earth is carefully concealed within the earth's crust.Thus, the geologic timescale and radiometric dating have been developed in an effort to determine the age of the earth.
For example, carbon dating is used to determine the age of organic materials.
Once something dies, it ceases taking in new carbon-14, and the existing carbon-14 within the organism decays into nitrogen at a fixed rate.
Relative dating observes the placement of fossils and rock in layers known as strata.
Basically, fossils and rock found in lower strata are older than those found in higher strata because lower objects must have been deposited first, while higher objects were deposited last.
William Smith was one of the most important scientists from this time who helped to develop knowledge of the succession of different fossils by studying their distribution through the sequence of sedimentary rocks in southern England.
It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.
Relative dating and radiometric dating are used to determine age of fossils and geologic features, but with different methods.
Relative dating uses observation of location within rock layers, while radiometric dating uses data from the decay of radioactive substances within an object.
Some very straightforward principles are used to determine the age of fossils.
Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.
This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.